At El Bulli, there is no hoax, no quaking charlatan pretending to be powerful. Ferran Adrià really is working wonders with food, the likes of which have never before been seen. The only trick – if that's the right word – is that he is not doing it alone. Behind the curtain, dozens of other cooks stand with him: in recent years, there have been about 45 people at a time working in El Bulli's kitchen, and since most of them stay for just one season, the total over the two decades since he began experimenting with food reaches high into the hundreds.

Each year, around 3,000 apply for the privilege of passing six months as stagiaires – technically the word means "interns" or "apprentices," but it translates metaphorically as "kitchen slaves"– at its stove tops. The high regard in which the restaurant is held, as well as the chance to receive instruction from Adrià and his chefs, explains why the final 32 apprentices travel, at their own expense, from everywhere from Seoul to Bologna and Los Angeles to Caracas, to the tiny, overbuilt town of Roses on Spain's Costa Brava. It is also why they agree to work unpaid for 14 hours at a stretch in exchange for one meal a day and a bed in an unattractive apartment. It is why they stand virtually unmoving for seven of those hours, feet planted at the centre counter, squeezing the germ from thousands of kernels of corn or trimming the slime off anemones. If they make it through the six months that El Bulli is open, they can say they have worked in the best restaurant in the world.

Selection is based more on instinct than anything resembling a formal hiring process. El Bulli does not interview prospective stagiaires; Marc Cuspinera, chef de cuisine, makes the call largely on the basis of how an applicant fills out a form. He is not swayed solely by enthusiasm; experience counts. He tends to prefer those who come from restaurants whose level he considers roughly comparable with El Bulli's – Per Se, Alinea, the Fat Duck – and younger cooks ("A 40-year-old won't last the six months," he says. "I know, I'm 40.")

On the first day, the stagiaires appear one by one on Roses' promenade, dragging their suitcases and backpacks behind them in the bright morning sunlight. Those who arrive first grab an outdoor table at the beachfront cafe, and order coffee or beer, depending on how they spent the previous night and how cocky they are feeling that morning. With each new arrival, the seated ones squint up into the blinding sun and offer a wary hello. It feels like the first day of a summer camp.

Soon all 32 are standing around El Bulli's stove tops with the 13 permanent staff, including Adrià. Dressed in sandals and jeans, his dark, curly hair beginning to grey, and his round belly straining against the confines of his T-shirt, Adrià is not an intimidating figure, at least not until he begins to speak. In his staccato voice, he delivers a speech that is half pep talk, half hellfire and brimstone.

He makes it clear that more than one tardy arrival will mean automatic dismissal. (In fact, no less famous a former El Bulli cook than José Andrés – today a chef and the owner of top restaurants in Washington and Los Angeles and a close friend of Adrià – was fired on the spot when Adrià arrived at the Barcelona bar where they had agreed to meet at a certain time, and discovered that Andrés had not yet arrived. The fact that Andrés had got there an hour early, after an eight-hour bus ride from Madrid, and stepped out to a phone booth to call Adrià, apparently made no difference.)

Mobile phones are prohibited in the kitchen and anyone caught taking photos will be fired – the restaurant has had more than one bad experience in which stagiaires have posted a new or developing dish on the internet. Then Adrià says something that comes as a surprise to those cooks who have worked in other highly regarded places: "I don't want to hear any screaming in this restaurant. No one insults anyone else, no one belittles anyone else. If you have a criticism, you raise it calmly, in a meeting. If I hear of anyone insulting anyone else, they're out."

Precision, physical endurance, the abilities to overcome disgust and keep one's opinions to oneself: these are all key qualities in a stagiaire. The work is bloody and mildly disgusting – though nothing is as bad as the rabbit brains. For those, the stagiaires are confronted with several trays of neatly arranged rabbit heads, their fur and skin removed to expose the pink flesh beneath, their tiny teeth bared, their unseeing eyes all staring in the same direction. Each cook picks up a head, positions it on his cutting board, and drives the tip of a chef's knife through the centre of the skull until it splits open. Picking up the half head, he carefully spoons out the cerebellum. Provided, of course, that he hasn't accidentally driven his knife too hard into the carapace. In that case, he will have splattered the brains all over the interior of the skull. "I never got used to that one," confesses Jorge Puerta, from Venezuela, who was once a vegetarian. "I just couldn't look at those rows of peeled bunny heads."

On the day the restaurant opens, Adrià begins the morning meeting by trying to calm everyone. "Tonight people will come from all over the world to eat our cooking. They expect it to be the best meal, even the best day, of their year." He pauses to look around at his new crew, whose faces have drained of all colour at this reminder. "But don't worry. The world won't end this week, no matter what happens in this restaurant. Relax. Have a good time."

At El Bulli, all diners are ushered into the kitchen before sitting down to eat. Later in the season, the stagiaires won't notice this part, not even when famous chefs such as Wolfgang Puck come to dine. But on this first night, as the first clients – an Australian couple who have dressed up for the occasion – are led into the kitchen, the cooks stop for a split second and gape. Chef de cuisine Eduard Xatruch's voice quickly breaks the trance. No sooner are the first couple on their way out of the kitchen than he calls, "Two coníferas," ordering the new pine-and-yoghurt cocktail that comes with a side of pine shoots to munch on. A few minutes later, "Start two menus," which is the signal to the kitchen that the table has finished its snacks and cocktails and moved on to the main part of the meal.

Nerves are on edge: as chef de partie Pablo Pavón goes to plate some delicate crêpes that another chef de partie, Toni Moraga, has prepared, his hand shakes. Adrià calls two more orders: a parmesan "crystal" to Mateu Casañas in Pastry and four bizcochos to Aitor Zabala in Cold Station (these bizcochos are his brother Albert's spectacular invention – an airy but substantive sponge cake made from miso and black sesame paste, aerated in the siphon and then baked in a microwave).

As each tray is readied to go out, Oriol Castro, the restaurant's second in command, Eugeni de Diego the sous chef, Xatruch and Cuspinera stand nearby, watching intently. One sends back a chicharrón that has been fried too long, another catches a plate with fingerprints on it. The noise level rises again, as cooks rush through the kitchen with cries of "Quemo! Quemo!" ("coming through, coming through"). Adrià, raising his voice, silences them.

By 8.50pm, when the last table shows up (it is Rafael Ansón, the Gastronomy Academy president, who always comes to dine on the first night of the season), the kitchen has descended into apparent chaos. It is only now that the stagiaires have grasped an essential point of El Bulli: during service, no one will be calmly telling them to put this element here, plate that component there. Instead, they have to listen to all the orders as they are called, figure out what station they'll be coming from, remember what goes on which dishes, then, worst of all, fight off their fellow stagiaires for a place at the plate. This first service looks like a rugby match, and a bloody one at that. Stagiaires with small cups of herbs elbow others with pipettes of oil out of the way in order to get to the plate, while still others, momentarily bereft of anything to do, crowd around trying to watch.

A server hurriedly picks up a tray, and in the brusqueness of the movement an orange segment falls from the plate to the floor. She looks around to see if anyone has noticed and continues on her way – there are a lot more segments on the plate. But just before she reaches the edge of the dining room, Xus González, one of the maître d's, calls her back; he has been watching the whole time.

"What are you doing?" he demands, before sending her back to the pass.

Chef de partie Aitor Zabala can't bring himself to look at her as he replates.

At 10pm, Cold Station is in the weeds – or in the puree, as they say in Spanish. Its dishes, the restaurant's most complicated, have to be timed perfectly to prevent them from melting. Adrià calls the first set of lulos, made with the tart pulp of a South American fruit, and Lucho comes running with the plate's garnish: shavings of frozen foie gras.

To ensure that the fatty liver stays icy, Zabala pours a bucket of liquid nitrogen over the plates, and clouds billow over the pass and out onto the main floor, momentarily transforming the kitchen into the set of a horror movie. Adrià hurries over. He yells at Zabala, "What are you doing putting on the foie before the lulos are ready?"

Zabala sends the foie shavings back to the freezer and starts the plates again. But he is only halfway through before Adrià is ordering nine yemitas, an Asian-inspired dish made with spherified egg yolks. Zabala throws down nine plates, and the entire station starts to work on a single order: Ralph Schelling squeezing out the yolks with a syringe, Jacobo spooning on the yoghurt gelee, Lucho adding a few drops of thickened tea, Zabala sprinkling toasted sesame seeds. It's taking them too long, and Castro runs over and jumps into the fray, "C'mon, c'mon, c'mon," he urges.

Zabala grabs the syringe out of Ralph's hand and starts squeezing yolks; Cuspinera comes over to wipe down the edges of plates. Finally, the dishes go out. But by then, of course, there are more lulos to plate.

At 10.45, Laia and Emma Leftick start rolling the first omelette surprise, a pre-dessert. Thirty minutes later, 12 stagiaires are at the centre table, nine of them rolling yuba around the yoghurt foam that three others spray from siphons. Adrià is displeased with what he finds and lays into Xatruch for not ensuring that the stagiaires have mastered the technique. Xatruch studiously avoids looking at him. But at 12.30, just as the main kitchen has finished with all its plates (Cold Station and Pastry are still hard at work), he calls an emergency training session. Chef de partie Luis Arrufat looks at the clock, looks at Xatruch to see if he is serious, and shakes his head. For the next half hour, the stagiaires, with 13 hours of mise en place and service behind them, practise rolling milk skins. No one says a word.

Finally they are allowed to clean down the kitchen. Each grabs a bucket of soapy water and attacks a different part: stove, floor, counter, walls. While the stagiaires clean, the chefs confer about the night. "That's one day down," Xatruch says. "Only 195 to go."